Surface-agnostic highly stretchable and bendable conductive MXene multilayers

Abstract

Stretchable, bendable, and foldable conductive coatings are crucial for wearable electronics and biometric sensors. These coatings should maintain functionality while simultaneously interfacing with different types of surfaces undergoing mechanical deformation. MXene sheets as conductive two-dimensional nanomaterials are promising for this purpose, but it is still extremely difficult to form surface-agnostic MXene coatings that can withstand extreme mechanical deformation. We report on conductive and conformal MXene multilayer coatings that can undergo large-scale mechanical deformation while maintaining a conductivity as high as 2000 S/m. MXene multilayers are successfully deposited onto flexible polymer sheets, stretchable poly(dimethylsiloxane), nylon fiber, glass, and silicon. The coating shows a recoverable resistance response to bending (up to 2.5-mm bending radius) and stretching (up to 40% tensile strain), which was leveraged for detecting human motion and topographical scanning. We anticipate that this discovery will allow for the implementation of MXene-based coatings onto mechanically deformable objects.

Fig. 1. Structural and morphological characterizations of MXene multilayers.

(A) Schematic of the PDAC/MXene LbL assembly process. Images of (B) immersion and (C) spray assembly of multilayer coatings of varying number of layer pairs on glass. (D) A cross-sectional SEM image of the multilayer coating. (E) Ultraviolet–visible (UV-vis) spectra of MXene multilayers on glass. (F) Absorbance values at 770 nm versus number of layer pairs. a.u., arbitrary units. (G) Growth profile of the multilayers on glass. (H) Root-mean-square (RMS) roughness versus number of layer pairs.

Fig. 2. Surface-agnostic conductive coatings.

(A) Digital images of 40-layer-pair coatings on various substrates (sheet resistance of coatings on slide glass, PDMS, PET, and kirigami PET: 7, 7, 4, and 4 kilohm per square, respectively). (B) A digital image and SEM images of bare nylon fiber and 20-layer-pair–coated nylon fiber. (C) Images to demonstrate conductive coating on nylon fiber (R = 26.5 megohm). (D) Sheet resistance of the MXene multilayers on glass. (E) Schematic illustration of an electric circuit with a battery, a light-emitting diode (LED), and the MXene multilayer (LbL film). Digital images to demonstrate the conductive coating on PET under bending and folding.

Fig. 3. Strain sensor behavior under bending and stretching.

(A) Normalized resistance (R/R₀) versus bending radius for 20-layer-pair MXene multilayer on PET and (B) versus strain for 20-layer-pair MXene multilayer on PDMS. R₀ = 22.4 kilohm (bending) and 1.66 megohm (stretching). Cycling performance under (C) bending and (D) stretching. SEM images of the surface structure of the 20-layer-pair MXene multilayer on (E) PET (bending) and (F) PDMS (stretching). The deformed coatings on PET and PDMS are under bending (r = 4.4 mm) and stretching (ε = 20%), respectively.

Fig. 4. An object scanner and human motion sensor.

(A) Digital image of a topographical scanner using the MXene multilayer–coated PET (LbL sensor). (B) The topographical map by normalized resistance variations with various cube patterns. (C) Topographical scanner with the five MXene multilayer–coated PET sensors and (D) T, A, M, and U patterns using cubes. (E) Topographical maps of normalized resistance variations for the T, A, M, and U patterns. (F) Digital image of the human motion strain sensor. (G) Response to finger motion.